This invention relates generally to methods and apparatus for acquiring the carrier frequency in a wireless communication system and specifically to a method and apparatus for examining the band edges of spectrally symmetric communications channels to iteratively derive the carrier frequency.
The need to quickly and accurately acquire the carrier frequency of a transmitted signal is well known in the art. Wireless communications (such as two-way radios, cellular telephones, and the like) transmit by modulating an information signal onto a carrier signal or carrier frequency. In order to accurately demodulate the transmitted signal at the receiver, the carrier frequency must be accurately detected and the receiver must match its clock (oscillator) as closely as possible to the received carrier frequency.
Typical radio communication systems have base units with highly accurate and stable clocks (oscillators). The mobile units, however, generally are designed to minimize cost, weight, and power consumption and hence typically employ oscillators and clock circuits that are not as accurate or stable as those found in base units. Therefore, even if the signal is always transmitted on a known, non-varying carrier frequency, the need exists at the receiver to acquire that frequency and to bring the mobile receiver oscillator into conformity with (i.e. match to) the carrier frequency.
In some prior art systems, pilot tones have been employed to assist in carrier acquisition. These pilot tones (or pilot signals) are generally unmodulated and provide a beacon in the received signal. The receiver expects the pilot tones to be certain pre-defined frequencies and can acquire the pilot tones, using frequency and phase locked loop feedback systems to lock onto the correct pilot tone, even if the receiver""s clock circuitry is initially off by some amount. The shortcoming with the use of pilot tones is that the pilot signals take up a portion of the transmission bandwidth, which is a limited and expensive resource.
Certain modulation and transmissions schemes, such as quadrature amplitude modulation (QAM) dispense entirely with the use of pilot tones. Such systems require another method for acquiring the carrier signal. Some such systems use phase locked loops to acquire the carrier signal. This method has a shortcoming in that in the event the base unit and mobile unit are significantly out of synch initially, the phase locked loop might lock onto an entirely incorrect frequency. Other prior art systems employ training symbols of a known configuration at the start of and periodically during transmission. These training symbols reduce the bandwidth available for the desired information transmission, however.
Therefore, a need exists in the art for a system for acquiring and locking onto a carrier frequency, which system does not introduce significant latency and does not require excessive computational power, which does not consume bandwidth from the desired information signal, and which is accurate across a wide range of mismatch between the transmitting and receiving units. The present invention provides such a solution, as will be described below with reference to certain preferred embodiments.
In one aspect, the present invention provides a method of deriving a carrier frequency for a communications channel of a pre-defined bandwidth and having an right and a left band edge, the carrier frequency being defined as the mid-point frequency of the pre-defined bandwidth. The method comprises deriving the right band edge spectral energy and the left band edge spectral energy at the right and left band edges of the channel, comparing the right band edge spectral energy and the left band edge spectral energy, and generating an error signal from said comparison step.
In another aspect, the invention provides a circuit for acquiring the carrier signal of a transmitted channel, the channel having an right and a left band edge. The circuit includes a first Goertzel discrete Fourier transform processor centered about the right band edge, a second Goertzel discrete Fourier transform processor centered about the left band edge, a comparator coupled to an output of said first Goertzel discrete Fourier transform processor and to an output of said second Goertzel discrete Fourier transform processor; and an error signal generator coupled to an output of said comparator. Embodiments of the invention may also include polarity detection circuit coupled to said output of said comparator and a voltage controlled oscillator coupled to an output of said error signal generator.
Features of the invention may be embodied in processes implemented on a digital signal processor (DSP), although the invention may also be embodied in integrated or discrete hardware components as well, or as a combination of DSP processes and hardware components.